Various image sensors capable of taking a 3D image of a subject with a monocular system have been proposed as disclosed in the following Patent Literatures 1, 2 and 3.
In an image sensor capable of taking a 3D image, for example, as shown in FIG. 27, a plurality of pixels arranged and formed in a two-dimensional array are divided into paired pixels 2 which are pairs of two adjacent pixels. Of each pair of the pixels, one pixel is provided with a light shielding film opening 2a made eccentric, for example, on the horizontal right side with respect to its pixel center, while the other pixel is provided with a light shielding film opening 2b made eccentric on the horizontally opposite side, that is, on the horizontally left side with respect to its pixel center.
The light shielding film opening 2a is eccentric on the right side so that incident light with which a right eye views a subject can be incident thereon. The light shielding film opening 2b is eccentric on the right side so that incident light with which a left eye views the subject can be incident thereon. Thus, an image taken by the pixels each having the light shielding film opening 2a is used as a right eye image and an image taken by the pixels each having the light shielding film opening 2b is used as a left eye image, so that a stereoscopic image (3D image) of the subject can be reproduced.
FIG. 28 is a graph showing sensitivities and incident angle dependencies of the paired pixels in FIG. 27. Here, zero degree in FIG. 28 designates an incident angle of zero degree, that is, an angle of incident light which is incident perpendicularly on the light receiving surface of a pixel. A plus side incident angle designates an incident angle from the direction of the light shielding film opening 2a which is made eccentric on the right side with respect to the pixel center. A minus side incident angle designates an incident angle from the direction of the light shielding film opening 2b which is made eccentric on the left side with respect to the pixel center. Sensitivity curves I and II of the pixels having the light shielding film openings 2a and 2b have incident angle dependencies respectively, and the difference in sensitivity between the paired pixels corresponds to a phase difference amount. As the ratio of the range (hatched range) where the two sensitivity curves I and II overlap each other to the total area is smaller, the phase difference amount between the two pixels increases. However, when the light shielding openings 2a and 2b are narrowed to reduce the overlapping range, the sensitivities become too low so that no image but a dark image can be taken. Therefore, the light shielding film openings 2a and 2b have to be set at suitable dimensions so that the light shielding film openings can be determined with suitable eccentricities.
Someone may want to take a 2D image of a subject even with an image sensor capable of taking a 3D image. When a 2D image is produced using detection signals of individual pixels of the image sensor shown in FIG. 27, the quality of the 2D image deteriorates due to a phase difference between a taken image signal of each pixel with the light shielding film opening 2a and a taken image signal of each pixel with the light shielding film opening 2b. 
Therefore, when the taken image signals of two paired pixels are added to each other in order to cancel the phase difference amount, a high-quality 2D image can be obtained. Due to the addition of the pixels, however, the resolution is halved with respect to the number of pixels provided in the image sensor. Thus, there arises a problem that a high-definition 2D image cannot be taken.